I can feel Ozzy's spine but he seems to be well proportioned. He is only 3lbs 12oz and is 7 months. My vet thinks he is just a peanut
I keep food down for him all day and now try and make sure he eats some lunch mid day. Try warming up the canned food before you give it to him. Ozzy hates cold dog food, (I don't blame him) , but will eat it if it's warmed up.
We dream about getting to 5lbs
(heck we'll take 4.5!!)